This invention relates to a printing apparatus for color electronic photographs and, more particularly, to a circuit in a printing apparatus for color electronic photographs for calculating laser intensity output to an engine part in the apparatus in which the number of registers required for representing the gradation curve is diminished to reduce the circuit size.
Heretofore, if a color picture is entered or output in the printing apparatus for color electronic photographs, the number of gradations per dot of input picture data needs to be converted into a number of gradations that can be represented by an engine portion prior to representation thereof. It is indispensable as the performance of a printing apparatus for the gradation of an output color picture in its entirety to maintain the gradation of the input picture.
Thus, picture processing termed half-tone processing is routinely performed. In this half-tone processing, the cell-to-cell angle (screen angle E) and the cell-to-cell distance (frequency, lines per inch) are adjusted depending on output engine characteristics, with a half-tone cell as a collection of a dot for representation as a unit. In the cell, the representing laser intensity is processed with gradation conversion by a pre-set gradation curve proper to the apparatus from dot to dot to give output data.
This processing, implemented in general by a printer description language in software, is not explained herein as to its detailed operation or processing. As an apparatus for generating a correction gradation curve for correcting gradation conversion characteristics of a reference gradation curve in order to effectuate radiation conversion for matching the gradation of picture data to the concentration gradation of a recording picture of a transcription paper sheet output by a printer on the basis of picture data obtained on reading an original picture having gradation characteristics, reference is made to the JP Patent Kokai JP-A-8-181863. There is also disclosed in JP Patent Kokai JP-A-9-224154 a picture data processing apparatus including first storage means for storage of an output gradation reference curve, a second storage unit for storing plural normalized gradation change curves, selection means for selecting one of the plural normalized gradation change curves, and calculation means for changing the output gradation curve based on a gradation change curve selected by the selection means to find values on an output gradation curve to be formulated by such change, as output picture data, in order to reduce the quantity of the information to be stored in the storage means and the capacity of the storage means.
If the halftone processing is to be implemented by hardware, there is required a register for holding the gradation curve information for converting the input data gradation into output data gradation in each of dots in the half-tone cell.
Assume that the number of dots making up a half-tone cell is D, the number of input gradation bits is KI and the number of output gradation is KO, the number of these registers is
Dxc3x97KIxc3x97KO (bits).
The result is that the more the number of input and output gradations for realization of variegated color representation, the more is the number of required registers, thus directly leading to an increased hardware cost.
Moreover, if the number of registers is increased, the time required for setting values in the registers is increased to lower the printing performance of the printing apparatus itself.
In view of the above problems, it is a principal object of the present invention to provide a color laser printing apparatus having the function of realizing half-tone processing by the hardware such as to evade cost rise or lowered performance.
According to a first aspect of the present invention, there is provided a novel printing apparatus for color electronic photographs aimed at accomplishing the above object. The printing apparatus generates the gradation curve information by calculation of basic gradation curve data and shifting quantity data used for shifting the basic gradation curve data to effect gradation conversion processing of picture data, to reduce the number of registers required for representing a gradation curve.
According to a second aspect, specifically, the printing apparatus for color electronic photographs has a function of inputting gradation data of plural bits concerning representation of a dot, a function of comparing input gradation data to a pre-formulated gradation table to effect gradation conversion and a structure for implementing these functions by a hardware. The printing apparatus includes (a) a set of registers capable of holding, as data, the gradation curve information necessary for gradation conversion which determines data (i.e., data of laser intensity or radiation time) desired to be output depending on values of input gradation data, (b) a gradation curve calculating circuit which finds the gradation curve information to be held on said set of registers based on calculation, and (c) a basic gradation curve register and a shifting quantity register, as a picture processing circuit. The basic gradation curve register holds basic gradation curve data entered to the gradation curve calculating circuit, while the shifting quantity register holds shifting quantity data which is the amount of shifting of said basic gradation curve data.
The calculation is typically based on addition of two systems of input data. The term xe2x80x9cshiftingxe2x80x9d is generally understood as parallel shifting. The shift quantity (or amount of shift) is an addend to be used for the calculation to provide a parallel shifting of the basic gradation curve defined by the basic gradation curve data.
An embodiment carrying out the present invention will be hereinafter explained. The picture processing apparatus has a hardware circuit for half-tone processing an input picture. This hardware circuit includes a gradation curve calculation unit (4 of FIG. 2) for generating the gradation curve information of reference by calculations, a basic gradation curve storage unit (1 of FIG. 2) for storing the information on the basic gradation curve required for the calculations; and a shifting quantity storage unit (2 of FIG. 2) for storing the shifting quantity information for shifting calculations of the basic gradation curve by the gradation curve calculation means. The basic gradation curve information and the shifting quantity information are set in registers, respectively, so that the gradation curve calculating unit (circuit) generates the gradation curve information required for half tone processing. By having reference to this gradation curve information, the input gradation data is processed by the calculating unit with gradation conversion processing to generate output gradation data which is output.
The above objects, features, operation and result will become apparent from the following description of the embodiment of the present invention.